libext2fs: verify and calculate extended attribute block checksums

libext2fs: verify and calculate extended attribute block checksums

Calculate and verify the checksum for separate (i.e. not in the inode)
extended attribute blocks; the checksum lives in the header.

[ Merged in change from Tao so that we always use the fs checksum seed
  for the xattr blocks. ]

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>